The reality is Teesside and Middlesbrough is closer to Sunderland and Sunderland supporting areas than large parts of Newcastle supporting Northumberland. The Geordie fans I know north of Newcastle don't even acknowledge Sunderland. They quite like Boro too as it happens. That said, the Tyne and Wear area is much more joined together than Wearside and Teesside, so it's natural that area feels more as one.
I must say I've got an opposite view to many regarding games against local sides. I'm North East England born and bred and like to see all the local teams do well. Travelling extensively throughout the area also gives me a different perspective of the area. I'm of the opinion that the big three sides should all be in the top flight together, as long as Boro win all the games of course.